This invention provides re-processable thermoplastic elastomer compositions comprising a thermoplastic polyurethane polymer and a silicone elastomer and methods for making them.
Thermoplastic elastomers (TPEs) are polymeric materials which possess both plastic and rubbery properties. They have elastomeric mechanical properties but, unlike conventional thermoset rubbers, they can be re-processed at elevated temperatures. This re-processability is a major advantage of TPEs over chemically crosslinked rubbers since it allows recycling of fabricated parts and results in a considerable reduction of scrap.
In general, two main types of thermoplastic elastomers are known. Block copolymer thermoplastic elastomers contain xe2x80x9chardxe2x80x9d plastic segments which have a melting point or glass transition temperature above ambient as well as xe2x80x9csoftxe2x80x9d polymeric segments which have a glass transition or melting point considerably below room temperature. In these systems, the hard segments aggregate to form distinct microphases and act as physical crosslinks for the soft phase, thereby imparting a rubbery character at room temperature. At elevated temperatures, the hard segments melt or soften and allow the copolymer to flow and to be processed like an ordinary thermoplastic resin.
Alternatively, a thermoplastic elastomer referred to as a simple blend (physical blend) can be obtained by uniformly mixing an elastomeric component with a thermoplastic resin. When the elastomeric component is also cross-linked during mixing, a thermoplastic elastomer known in the art as a thermoplastic vulcanizate (TPV) results. Since the crosslinked elastomeric phase of a TPV is insoluble and non-flowable at elevated temperature, TPVs generally exhibit improved oil and solvent resistance as well as reduced compression set relative to the simple blends.
Typically, a TPV is formed by a process known as dynamic vulcanization, wherein the elastomer and the thermoplastic matrix are mixed and the elastomer is cured with the aid of a crosslinking agent and/or catalyst during the mixing process. A number of such TPVs are known in the art, including some wherein the crosslinked elastomeric component can be a silicone polymer while the thermoplastic component is an organic, non-silicone polymer (i.e., a thermoplastic silicone vulcanizate).
Polyurethanes are an important class of thermoplastics finding utility in a variety of commercial applications. The physical properties of polyurethanes typically can be adjusted for various applications through the selection of the type and amount of starting materials (for example polyol, isocyanate, and chain extender) used in the composition. Alternatively, compounding polyurethanes with other polymers or materials can alter their physical properties.
Several attempts have been made to combine polyurethane with silicones to create unique compositions. U.S. Pat. No. 4,647,643 discloses, for example, soft non-blocking polyurethanes which are prepared by reacting a long chain polyester or polyether diol, a short chain diol, a diisocyanate and a silicone diol.
Arkles, in U.S. Pat. No. 4,500,688, discloses semi-interpenetrating networks (semi-IPNs) wherein a vinyl-containing silicone fluid having a viscosity of 500 to 100,000 cS is dispersed in a conventional thermoplastic resin. Typical thermoplastics mentioned include polyesters, polyurethanes, styrenics, polyacetals and polycarbonates. Arkles only illustrates these IPNs at relatively low levels of silicone. The vinyl-containing silicone is vulcanized in the thermoplastic during melt mixing according to a chain extension or crosslinking mechanism which employs a silicon hydride-containing silicone component. This disclosure is expanded by Arkles in U.S. Pat. No. 4,714,739 to include the use of hybrid silicones which contain unsaturated groups and are prepared by reacting a hydride-containing silicone with an organic polymer having unsaturated functionality. Although Arkles discloses a silicone fluid content ranging from 1 to 40 weight percent (1 to 60% in the case of the ""739 patent), there is no suggestion of any criticality as to these proportions or to the specific nature of the organic resin. Furthmore, Arkles provides no teaching on how to improve the physical properties of polyurethanes upon long term exposure to heat.
Thermoplastic silicone vulcanizates (TPSiVs as discussed supra) have been prepared by condensation cure of a thermoplastic resin and a silanol-terminated diorganopolysiloxanes in a dynamic vulcanization process, as disclosed in U.S. Pat. No. 6,153,691. While the thermoplastic resin of the ""691 patent included polyurethanes, no specific polyurethane based compositions having improved physical properties are taught therein.
U.S. Pat. No. 4,164,491 discloses thermally curable silicone rubber compositions comprising diorganopolysiloxanes gum, a polyurethane elastomer having a softening point of not lower than 100xc2x0 C., a reinforcing filler, and an organic peroxide. However, the resulting cured products from the ""491 patent are taught to be xe2x80x9csilicone rubberxe2x80x9d and thus not re-processable.
Despite these advances in the art of polyurethane compositions, a need still exists to further identify polyurethane compositions with improved physical properties. In particular, there is a need for polyurethane compositions with lower durometer values (hardness) without sacrificing overall strength. Additionally, there is a need to identify polyurethane compositions that retain physical property profiles after exposure to heat (for example at 120 -150xc2x0 C.) for extended periods of time. Furthermore, there is a need to identify such polyurethane compositions that are re-processable.
The present inventors have discovered re-processable thermoplastic elastomer compositions that are produced in a dynamic vulcanization process from a thermoplastic polyurethane polymer and a silicone elastomer. The compositions of the present invention possess unique physical properties vs previously disclosed polyurethane-silicone compositions. Furthermore, many physical properties, such as hardness, tensile strength, elongation, and compression set remain similar, or degrade little, when the compositions are exposed to heat for extended periods of time.
The present invention is directed to a thermoplastic elastomer composition comprising;
(A) a thermoplastic polyurethane polymer,
(B) a silicone elastomer,
wherein the weight ratio of the silicone elastomer to the thermoplastic polyurethane polymer is from 5:95 to 85:15, and the thermoplastic elastomer composition is re-processable.
The invention also provides a method making thermoplastic elastomer compositions comprising:
(I) mixing
(A) a thermoplastic polyurethane polymer,
(B) a silicone base comprising;
(Bxe2x80x2) 100 parts by weight of a diorganopolysiloxane gum having a plasticity of at
least 30 and having an average of at least 2 alkenyl groups in its molecule and, optionally,
(Bxe2x80x3) up to 200 parts by weight of a reinforcing filler,
the weight ratio of said silicone elastomer to said thermoplastic polyurethane resin is from 5:95 to 85:15,
(C) an organohydrido silicon compound which contains an average of at least 2 silicon-bonded hydrogen groups in its molecule and
(D) a hydrosilation catalyst,
components (C) and (D) being present in an amount sufficient to cure said diorganopolysiloxane (Bxe2x80x2); and
(II) dynamically vulcanizing said diorganopolysiloxane (Bxe2x80x2).